1. Field of the Invention
The present invention relates generally to methods and apparatus for carrying on simultaneous and unrelated transmissions over a single optical fiber. For example, TV signals may be transmitted downstream at 1550 nanometers of light and telephony signals may be transmitted bidirectionally 1310 nanometers of light. More specifically the invention relates to methods and apparatus for use with bidirectional telephony communications using two different codes such as NRZ (non-return to zero) coding and Manchester coding for upstream and downstream transmissions.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
The communications industry is using more and more optical or light fibers in lieu of copper wire. Optical fibers have an extremely high bandwidth thereby allowing significantly more information than can be carried by a copper wire transmission line such as twisted pairs or coaxial cable.
Of course, modern telephone systems require bidirectional communications where each station or user on a communication channel can both transmit and receive. This is true, of course, whether using electrical wiring or optical fibers as the transmission medium. Early telephone communication systems solved this need by simply providing separate copper wires for carrying the communications in each direction, and this approach may still be used in part of the transmission path. It is used to a greater degree as the signals get closer to the home or business end users. Although twisted pairs and coaxial cables are used in homes and distribution terminals close to the end user, some modern telecommunication systems now use micro-wave and optic fibers as transmission mediums. In addition various techniques are often used in optical transmission so that a signal optical fiber can carry communications in both direction.
However, because of extremely high band widths available for use by an optical fiber, a single fiber is quite capable of carrying a great number of communications in both directions. One technique of optical transmission is WDM (wavelength divisional multiplexing) which uses different wavelengths for different types of transmissions. Typical examples are the use of 1550 nanometers of light for TV signals transmission and 1310 nanometers of light for bidirectional telephony transmission.
It is noted that the term telephony is used rather than telephone to underscore the fact that communication transmission will include vocal telephone use but is not so limited. Typical telephony systems operate at a single frequency or wavelength of light which is divided into upstream and downstream carefully synchronized time windows. Telephony systems may use a single optical fiber and often even a single diode, for both converting electrical signals to optical signals and converting received optical signals to electrical signals.
However, as mentioned above, optical fibers have extremely high band widths and use of an optical fiber for a single telephone channel is a very ineffective use of the fiber and, in fact, the available bandwidth of an optical fiber is what makes it possible to use two different transmission techniques such as bidirectional telephone techniques at one frequency and another technique at a second frequency. Typically, two frequencies regardless of the two techniques being used are combined by the use of WDM technology.
A major problem for the bidirectional telephony signals is light reflection typically occurring at optical connections or interfaces along the optical fiber, and in a worse case situation, the reflected energy may be intercepted as an actual signal transmission in the bidirectional communication. Therefore, a technique for transmitting bidirectional telephony signals having minimal energy overlap and at difference frequencies would make efficient and effective use of an optical fiber.